Conventional electronic video cameras utilize charge-coupled device ("CCD") imagers to convert optical images into corresponding electronic signals. A typical CCD imager includes a matrix of photosites that are capable of collecting free electrons that form an electrical charge packet that is directly proportional to the photon radiation incident at that photosite. Accordingly, when an image is focused on the surface of the array, the charge packet at each photosite of the array corresponds to a respective picture element or pixel of the complete image. The generated photosite charge packets are transferred in a parallel manner into a CCD shift register, where they are transferred serially as an output signal of the CCD imager. The CCD imager output signal is a series of electronic signals whose amplitudes represent the light intensity of the image at each pixel for a single image frame. The process is continuously repeated to generate output signals which correspond to several image frames per second. As a consequence, each frame in the generated output signal contains information concerning the detected light intensity at each photosite.
A drawback of typical CCD imagers is that they require near-perfect charge transfer over distances on the order of centimeters through semiconductor materials. As a consequence, such CCD imagers require a substantial amount of power and are difficult to use under low light conditions, difficult to manufacture in large array sizes, and difficult to integrate with on-chip electronics. Active pixel sensors have become increasingly popular as an alternative to CCD imagers in video equipment. Active pixel sensors employ one or more on-chip transistors at each pixel photosite in the array. The transistors buffer a photo-signal corresponding to the charge packet generated by the photosite and drive an output signal line avoiding the disadvantages of the CCD imager. However, typical active pixel sensor arrays still generate an output signal with each frame containing information concerning the detected light intensity at each photosite.
In certain applications, such as motion detection for use in, for example, security cameras, and data compression for high-definition television ("HDTV"), it is desirable to generate a video signal that corresponds to only the difference between two adjacent frames rather than the actual detected light intensity at the pixels for each of the frames themselves. Accordingly, if there is no motion or change in a scene viewed by such system, then the system generates no output signal. If, however, an inter-frame change appears due to motion or a change in the scene, then the imaging system generates a corresponding differential output signal. A conventional CCD imager and known active pixel sensor arrays require additional equipment to generate a differential output signal. Such equipment, which may include an analog-to-digital converter, a frame buffer memory and a system controller, increases the complexity, cost, power consumption and size of the system.
As is apparent from the above, a need exists for a low power, compact imaging system having a differential output signal.